Proyectos de Subversion Moodle

<?php

// This file is part of Moodle - http://moodle.org/

//

// Moodle is free software: you can redistribute it and/or modify

// it under the terms of the GNU General Public License as published by

// the Free Software Foundation, either version 3 of the License, or

// (at your option) any later version.

//

// Moodle is distributed in the hope that it will be useful,

// but WITHOUT ANY WARRANTY; without even the implied warranty of

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

// GNU General Public License for more details.

//

// You should have received a copy of the GNU General Public License

// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**

 * Scheduled and adhoc task management.

 *

 * @package    core

 * @category   task

 * @copyright  2013 Damyon Wiese

 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later

 */

namespace core\task;

use core\lock\lock;

use core\lock\lock_factory;

use core_shutdown_manager;

define('CORE_TASK_TASKS_FILENAME', 'db/tasks.php');

/**

 * Collection of task related methods.

 *

 * Some locking rules for this class:

 * All changes to scheduled tasks must be protected with both - the global cron lock and the lock

 * for the specific scheduled task (in that order). Locks must be released in the reverse order.

 * @copyright  2013 Damyon Wiese

 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later

 */

class manager {

    /**

     * @var int Used to tell the adhoc task queue to fairly distribute tasks.

     */

    const ADHOC_TASK_QUEUE_MODE_DISTRIBUTING = 0;

    /**

     * @var int Used to tell the adhoc task queue to try and fill unused capacity.

     */

    const ADHOC_TASK_QUEUE_MODE_FILLING = 1;

    /**

     * @var int Used to set the retention period for adhoc tasks that have failed and to be cleaned up.

     * The number is in week unit. The default value is 4 weeks.

     */

    const ADHOC_TASK_FAILED_RETENTION = 4 * WEEKSECS;

    /**

     * @var ?task_base $runningtask Used to tell what is the current running task in this process.

     */

    public static ?task_base $runningtask = null;

    /**

     * @var bool Used to tell if the manager's shutdown callback has been registered.

     */

    public static bool $registeredshutdownhandler = false;

    /**

     * @var array A cached queue of adhoc tasks

     */

    protected static array $miniqueue = [];

    /**

     * @var int The last recorded number of unique adhoc tasks.

     */

    protected static int $numtasks = 0;

    /**

     * @var null|int Used to determine if the adhoc task queue is distributing or filling capacity.

     */

    protected static ?int $mode = null;

    /**

     * Reset the state of the task manager.

     */

    public static function reset_state(): void {

        self::$miniqueue = [];

        self::$numtasks = 0;

        self::$mode = null;

    }

    /**

     * Given a component name, will load the list of tasks in the db/tasks.php file for that component.

     *

     * @param string $componentname - The name of the component to fetch the tasks for.

     * @param bool $expandr - if true (default) an 'R' value in a time is expanded to an appropriate int.

     *      If false, they are left as 'R'

     * @return \core\task\scheduled_task[] - List of scheduled tasks for this component.

     */

    public static function load_default_scheduled_tasks_for_component($componentname, $expandr = true) {

        $dir = \core_component::get_component_directory($componentname);

        if (!$dir) {

            return array();

        }

        $file = $dir . '/' . CORE_TASK_TASKS_FILENAME;

        if (!file_exists($file)) {

            return array();

        }

        $tasks = null;

        include($file);

        if (!isset($tasks)) {

            return array();

        }

        $scheduledtasks = array();

        foreach ($tasks as $task) {

            $record = (object) $task;

            $scheduledtask = self::scheduled_task_from_record($record, $expandr, false);

            // Safety check in case the task in the DB does not match a real class (maybe something was uninstalled).

            if ($scheduledtask) {

                $scheduledtask->set_component($componentname);

                $scheduledtasks[] = $scheduledtask;

            }

        }

        return $scheduledtasks;

    }

    /**

     * Update the database to contain a list of scheduled task for a component.

     * The list of scheduled tasks is taken from @load_scheduled_tasks_for_component.

     * Will throw exceptions for any errors.

     *

     * @param string $componentname - The frankenstyle component name.

     */

    public static function reset_scheduled_tasks_for_component($componentname) {

        global $DB;

        $tasks = self::load_default_scheduled_tasks_for_component($componentname);

        $validtasks = array();

        foreach ($tasks as $taskid => $task) {

            $classname = self::get_canonical_class_name($task);

            $validtasks[] = $classname;

            if ($currenttask = self::get_scheduled_task($classname)) {

                if ($currenttask->is_customised()) {

                    // If there is an existing task with a custom schedule, do not override it.

                    continue;

                }

                // Update the record from the default task data.

                self::configure_scheduled_task($task);

            } else {

                // Ensure that the first run follows the schedule.

                $task->set_next_run_time($task->get_next_scheduled_time());

                // Insert the new task in the database.

                $record = self::record_from_scheduled_task($task);

                $DB->insert_record('task_scheduled', $record);

            }

        }

        // Delete any task that is not defined in the component any more.

        $sql = "component = :component";

        $params = array('component' => $componentname);

        if (!empty($validtasks)) {

            list($insql, $inparams) = $DB->get_in_or_equal($validtasks, SQL_PARAMS_NAMED, 'param', false);

            $sql .= ' AND classname ' . $insql;

            $params = array_merge($params, $inparams);

        }

        $DB->delete_records_select('task_scheduled', $sql, $params);

    }

    /**

     * Checks if the task with the same classname, component and customdata is already scheduled

     *

     * @param adhoc_task $task

     * @return bool

     */

    protected static function task_is_scheduled($task) {

        return false !== self::get_queued_adhoc_task_record($task);

    }

    /**

     * Checks if the task with the same classname, component and customdata is already scheduled

     *

     * @param adhoc_task $task

     * @return \stdClass|false

     */

    protected static function get_queued_adhoc_task_record($task) {

        global $DB;

        $record = self::record_from_adhoc_task($task);

        $params = [$record->classname, $record->component, $record->customdata];

        $sql = 'classname = ? AND component = ? AND ' .

            $DB->sql_compare_text('customdata', \core_text::strlen($record->customdata) + 1) . ' = ?';

        if ($record->userid) {

            $params[] = $record->userid;

            $sql .= " AND userid = ? ";

        }

        return $DB->get_record_select('task_adhoc', $sql, $params);

    }

    /**

     * Schedule a new task, or reschedule an existing adhoc task which has matching data.

     *

     * Only a task matching the same user, classname, component, and customdata will be rescheduled.

     * If these values do not match exactly then a new task is scheduled.

     *

     * @param \core\task\adhoc_task $task - The new adhoc task information to store.

     * @since Moodle 3.7

     */

    public static function reschedule_or_queue_adhoc_task(adhoc_task $task): void {

        global $DB;

        if ($existingrecord = self::get_queued_adhoc_task_record($task)) {

            // Only update the next run time if it is explicitly set on the task.

            $nextruntime = $task->get_next_run_time();

            if ($nextruntime && ($existingrecord->nextruntime != $nextruntime)) {

                $DB->set_field('task_adhoc', 'nextruntime', $nextruntime, ['id' => $existingrecord->id]);

            }

        } else {

            // There is nothing queued yet. Just queue as normal.

            self::queue_adhoc_task($task);

        }

    }

    /**

     * Queue an adhoc task to run in the background.

     *

     * @param \core\task\adhoc_task $task - The new adhoc task information to store.

     * @param bool $checkforexisting - If set to true and the task with the same user, classname, component and customdata

     *     is already scheduled then it will not schedule a new task. Can be used only for ASAP tasks.

     * @return boolean - True if the config was saved.

     */

    public static function queue_adhoc_task(adhoc_task $task, $checkforexisting = false) {

        global $DB;

        if ($userid = $task->get_userid()) {

            // User found. Check that they are suitable.

            \core_user::require_active_user(\core_user::get_user($userid, '*', MUST_EXIST), true, true);

        }

        $record = self::record_from_adhoc_task($task);

        // Schedule it immediately if nextruntime not explicitly set.

        if (!$task->get_next_run_time()) {

            $record->nextruntime = time() - 1;

        }

        // Check if the task is allowed to be retried or not.

        $record->attemptsavailable = $task->retry_until_success() ? $record->attemptsavailable : 1;

        // Set the time the task was created.

        $record->timecreated = time();

        // Check if the same task is already scheduled.

        if ($checkforexisting && self::task_is_scheduled($task)) {

            return false;

        }

        // Queue the task.

        $result = $DB->insert_record('task_adhoc', $record);

        return $result;

    }

    /**

     * Change the default configuration for a scheduled task.

     * The list of scheduled tasks is taken from {@link load_scheduled_tasks_for_component}.

     *

     * @param \core\task\scheduled_task $task - The new scheduled task information to store.

     * @return boolean - True if the config was saved.

     */

    public static function configure_scheduled_task(scheduled_task $task) {

        global $DB;

        $classname = self::get_canonical_class_name($task);

        $original = $DB->get_record('task_scheduled', array('classname'=>$classname), 'id', MUST_EXIST);

        $record = self::record_from_scheduled_task($task);

        $record->id = $original->id;

        $record->nextruntime = $task->get_next_scheduled_time();

        unset($record->lastruntime);

        $result = $DB->update_record('task_scheduled', $record);

        return $result;

    }

    /**

     * Utility method to create a DB record from a scheduled task.

     *

     * @param \core\task\scheduled_task $task

     * @return \stdClass

     */

    public static function record_from_scheduled_task($task) {

        $record = new \stdClass();

        $record->classname = self::get_canonical_class_name($task);

        $record->component = $task->get_component();

        $record->customised = $task->is_customised();

        $record->lastruntime = $task->get_last_run_time();

        $record->nextruntime = $task->get_next_run_time();

        $record->faildelay = $task->get_fail_delay();

        $record->hour = $task->get_hour();

        $record->minute = $task->get_minute();

        $record->day = $task->get_day();

        $record->dayofweek = $task->get_day_of_week();

        $record->month = $task->get_month();

        $record->disabled = $task->get_disabled();

        $record->timestarted = $task->get_timestarted();

        $record->hostname = $task->get_hostname();

        $record->pid = $task->get_pid();

        return $record;

    }

    /**

     * Utility method to create a DB record from an adhoc task.

     *

     * @param \core\task\adhoc_task $task

     * @return \stdClass

     */

    public static function record_from_adhoc_task($task) {

        $record = new \stdClass();

        $record->classname = self::get_canonical_class_name($task);

        $record->id = $task->get_id();

        $record->component = $task->get_component();

        $record->nextruntime = $task->get_next_run_time();

        $record->faildelay = $task->get_fail_delay();

        $record->customdata = $task->get_custom_data_as_string();

        $record->userid = $task->get_userid();

        $record->timestarted = $task->get_timestarted();

        $record->hostname = $task->get_hostname();

        $record->pid = $task->get_pid();

        $record->attemptsavailable = $task->get_attempts_available();

        return $record;

    }

    /**

     * Utility method to create an adhoc task from a DB record.

     *

     * @param \stdClass $record

     * @return \core\task\adhoc_task

     * @throws \moodle_exception

     */

    public static function adhoc_task_from_record($record) {

        $classname = self::get_canonical_class_name($record->classname);

        if (!class_exists($classname)) {

            throw new \moodle_exception('invalidtaskclassname', '', '', $record->classname);

        }

        $task = new $classname;

        if (isset($record->nextruntime)) {

            $task->set_next_run_time($record->nextruntime);

        }

        if (isset($record->id)) {

            $task->set_id($record->id);

        }

        if (isset($record->component)) {

            $task->set_component($record->component);

        }

        if (isset($record->faildelay)) {

            $task->set_fail_delay($record->faildelay);

        }

        if (isset($record->customdata)) {

            $task->set_custom_data_as_string($record->customdata);

        }

        if (isset($record->userid)) {

            $task->set_userid($record->userid);

        }

        if (isset($record->timestarted)) {

            $task->set_timestarted($record->timestarted);

        }

        if (isset($record->hostname)) {

            $task->set_hostname($record->hostname);

        }

        if (isset($record->pid)) {

            $task->set_pid($record->pid);

        }

        if (isset($record->attemptsavailable)) {

            $task->set_attempts_available($record->attemptsavailable);

        }

        return $task;

    }

    /**

     * Utility method to create a task from a DB record.

     *

     * @param \stdClass $record

     * @param bool $expandr - if true (default) an 'R' value in a time is expanded to an appropriate int.

     *      If false, they are left as 'R'

     * @param bool $override - if true loads overridden settings from config.

     * @return \core\task\scheduled_task|false

     */

    public static function scheduled_task_from_record($record, $expandr = true, $override = true) {

        $classname = self::get_canonical_class_name($record->classname);

        if (!class_exists($classname)) {

            debugging("Failed to load task: " . $classname, DEBUG_DEVELOPER);

            return false;

        }

        /** @var \core\task\scheduled_task $task */

        $task = new $classname;

        if ($override) {

            // Update values with those defined in the config, if any are set.

            $record = self::get_record_with_config_overrides($record);

        }

        if (isset($record->lastruntime)) {

            $task->set_last_run_time($record->lastruntime);

        }

        if (isset($record->nextruntime)) {

            $task->set_next_run_time($record->nextruntime);

        }

        if (isset($record->customised)) {

            $task->set_customised($record->customised);

        }

        if (isset($record->component)) {

            $task->set_component($record->component);

        }

        if (isset($record->minute)) {

            $task->set_minute($record->minute, $expandr);

        }

        if (isset($record->hour)) {

            $task->set_hour($record->hour, $expandr);

        }

        if (isset($record->day)) {

            $task->set_day($record->day);

        }

        if (isset($record->month)) {

            $task->set_month($record->month);

        }

        if (isset($record->dayofweek)) {

            $task->set_day_of_week($record->dayofweek, $expandr);

        }

        if (isset($record->faildelay)) {

            $task->set_fail_delay($record->faildelay);

        }

        if (isset($record->disabled)) {

            $task->set_disabled($record->disabled);

        }

        if (isset($record->timestarted)) {

            $task->set_timestarted($record->timestarted);

        }

        if (isset($record->hostname)) {

            $task->set_hostname($record->hostname);

        }

        if (isset($record->pid)) {

            $task->set_pid($record->pid);

        }

        $task->set_overridden(self::scheduled_task_has_override($classname));

        return $task;

    }

    /**

     * Given a component name, will load the list of tasks from the scheduled_tasks table for that component.

     * Do not execute tasks loaded from this function - they have not been locked.

     * @param string $componentname - The name of the component to load the tasks for.

     * @return \core\task\scheduled_task[]

     */

    public static function load_scheduled_tasks_for_component($componentname) {

        global $DB;

        $tasks = array();

        // We are just reading - so no locks required.

        $records = $DB->get_records('task_scheduled', array('component' => $componentname), 'classname', '*', IGNORE_MISSING);

        foreach ($records as $record) {

            $task = self::scheduled_task_from_record($record);

            // Safety check in case the task in the DB does not match a real class (maybe something was uninstalled).

            if ($task) {

                $tasks[] = $task;

            }

        }

        return $tasks;

    }

    /**

     * This function load the scheduled task details for a given classname.

     *

     * @param string $classname

     * @return \core\task\scheduled_task or false

     */

    public static function get_scheduled_task($classname) {

        global $DB;

        $classname = self::get_canonical_class_name($classname);

        // We are just reading - so no locks required.

        $record = $DB->get_record('task_scheduled', array('classname'=>$classname), '*', IGNORE_MISSING);

        if (!$record) {

            return false;

        }

        return self::scheduled_task_from_record($record);

    }

    /**

     * This function load the adhoc tasks for a given classname.

     *

     * @param string $classname

     * @param bool $failedonly

     * @param bool $skiprunning do not return tasks that are in the running state

     * @return array

     */

    public static function get_adhoc_tasks(string $classname, bool $failedonly = false, bool $skiprunning = false): array {

        global $DB;

        $conds[] = 'classname = ?';

        $params[] = self::get_canonical_class_name($classname);

        if ($failedonly) {

            $conds[] = 'faildelay > 0';

        }

        if ($skiprunning) {

            $conds[] = 'timestarted IS NULL';

        }

        // We are just reading - so no locks required.

        $sql = 'SELECT * FROM {task_adhoc}';

        if ($conds) {

            $sql .= ' WHERE '.implode(' AND ', $conds);

        }

        $rs = $DB->get_records_sql($sql, $params);

        return array_map(function($record) {

            return self::adhoc_task_from_record($record);

        }, $rs);

    }

    /**

     * This function returns adhoc tasks summary per component classname

     *

     * @return array

     */

    public static function get_adhoc_tasks_summary(): array {

        global $DB;

        $now = time();

        $records = $DB->get_records('task_adhoc');

        $summary = [];

        foreach ($records as $r) {

            if (!isset($summary[$r->component])) {

                $summary[$r->component] = [];

            }

            if (isset($summary[$r->component][$r->classname])) {

                $classsummary = $summary[$r->component][$r->classname];

            } else {

                $classsummary = [

                    'nextruntime' => null,

                    'count' => 0,

                    'failed' => 0,

                    'running' => 0,

                    'due' => 0,

                    'stop' => false,

                ];

            }

            $classsummary['count']++;

            $nextruntime = (int)$r->nextruntime;

            if (!$classsummary['nextruntime'] || $nextruntime < $classsummary['nextruntime']) {

                $classsummary['nextruntime'] = $nextruntime;

            }

            if ((int)$r->timestarted > 0) {

                $classsummary['running']++;

            } else {

                if ((int)$r->faildelay > 0) {

                    $classsummary['failed']++;

                }

                if ($nextruntime <= $now) {

                    $classsummary['due']++;

                }

            }

            // Mark the task as stopped if it has no attempts available.

            if (isset($r->attemptsavailable) && $r->attemptsavailable == 0) {

                $classsummary['stop'] = true;

            }

            $summary[$r->component][$r->classname] = $classsummary;

        }

        return $summary;

    }

    /**

     * This function load the default scheduled task details for a given classname.

     *

     * @param string $classname

     * @param bool $expandr - if true (default) an 'R' value in a time is expanded to an appropriate int.

     *      If false, they are left as 'R'

     * @return \core\task\scheduled_task|false

     */

    public static function get_default_scheduled_task($classname, $expandr = true) {

        $task = self::get_scheduled_task($classname);

        $componenttasks = array();

        // Safety check in case no task was found for the given classname.

        if ($task) {

            $componenttasks = self::load_default_scheduled_tasks_for_component(

                    $task->get_component(), $expandr);

        }

        foreach ($componenttasks as $componenttask) {

            if (get_class($componenttask) == get_class($task)) {

                return $componenttask;

            }

        }

        return false;

    }

    /**

     * This function will return a list of all the scheduled tasks that exist in the database.

     *

     * @return \core\task\scheduled_task[]

     */

    public static function get_all_scheduled_tasks() {

        global $DB;

        $records = $DB->get_records('task_scheduled', null, 'component, classname', '*', IGNORE_MISSING);

        $tasks = array();

        foreach ($records as $record) {

            $task = self::scheduled_task_from_record($record);

            // Safety check in case the task in the DB does not match a real class (maybe something was uninstalled).

            if ($task) {

                $tasks[] = $task;

            }

        }

        return $tasks;

    }

    /**

     * This function will return a list of all adhoc tasks that have a faildelay

     *

     * @param int $delay filter how long the task has been delayed

     * @return \core\task\adhoc_task[]

     */

    public static function get_failed_adhoc_tasks(int $delay = 0): array {

        global $DB;

        $tasks = [];

        $records = $DB->get_records_sql('SELECT * from {task_adhoc} WHERE faildelay > ?', [$delay]);

        foreach ($records as $record) {

            try {

                $tasks[] = self::adhoc_task_from_record($record);

            } catch (\moodle_exception $e) {

                debugging("Failed to load task: $record->classname", DEBUG_DEVELOPER, $e->getTrace());

            }

        }

        return $tasks;

    }

    /**

     * Ensure quality of service for the ad hoc task queue.

     *

     * This reshuffles the adhoc tasks queue to balance by type to ensure a

     * level of quality of service per type, while still maintaining the

     * relative order of tasks queued by timestamp.

     *

     * @param array $records array of task records

     * @param array $records array of same task records shuffled

     * @deprecated since Moodle 4.1 MDL-67648 - please do not use this method anymore.

     * @todo MDL-74843 This method will be deleted in Moodle 4.5

     * @see \core\task\manager::get_next_adhoc_task

     */

    public static function ensure_adhoc_task_qos(array $records): array {

        debugging('The method \core\task\manager::ensure_adhoc_task_qos is deprecated.

             Please use \core\task\manager::get_next_adhoc_task instead.', DEBUG_DEVELOPER);

        $count = count($records);

        if ($count == 0) {

            return $records;

        }

        $queues = []; // This holds a queue for each type of adhoc task.

        $limits = []; // The relative limits of each type of task.

        $limittotal = 0;

        // Split the single queue up into queues per type.

        foreach ($records as $record) {

            $type = $record->classname;

            if (!array_key_exists($type, $queues)) {

                $queues[$type] = [];

            }

            if (!array_key_exists($type, $limits)) {

                $limits[$type] = 1;

                $limittotal += 1;

            }

            $queues[$type][] = $record;

        }

        $qos = []; // Our new queue with ensured quality of service.

        $seed = $count % $limittotal; // Which task queue to shuffle from first?

        $move = 1; // How many tasks to shuffle at a time.

        do {

            $shuffled = 0;

            // Now cycle through task type queues and interleaving the tasks

            // back into a single queue.

            foreach ($limits as $type => $limit) {

                // Just interleaving the queue is not enough, because after

                // any task is processed the whole queue is rebuilt again. So

                // we need to deterministically start on different types of

                // tasks so that *on average* we rotate through each type of task.

                //

                // We achieve this by using a $seed to start moving tasks off a

                // different queue each time. The seed is based on the task count

                // modulo the number of types of tasks on the queue. As we count

                // down this naturally cycles through each type of record.

                if ($seed < 1) {

                    $shuffled = 1;

                    $seed += 1;

                    continue;

                }

                $tasks = array_splice($queues[$type], 0, $move);

                $qos = array_merge($qos, $tasks);

                // Stop if we didn't move any tasks onto the main queue.

                $shuffled += count($tasks);

            }

            // Generally the only tasks that matter are those that are near the start so

            // after we have shuffled the first few 1 by 1, start shuffling larger groups.

            if (count($qos) >= (4 * count($limits))) {

                $move *= 2;

            }

        } while ($shuffled > 0);

        return $qos;

    }

    /**

     * This function will dispatch the next adhoc task in the queue. The task will be handed out

     * with an open lock - possibly on the entire cron process. Make sure you call either

     * {@link adhoc_task_failed} or {@link adhoc_task_complete} to release the lock and reschedule the task.

     *

     * @param int $timestart

     * @param bool $checklimits Should we check limits?

     * @param string|null $classname Return only task of this class

     * @return \core\task\adhoc_task|null

     * @throws \moodle_exception

     */

    public static function get_next_adhoc_task(int $timestart, ?bool $checklimits = true, ?string $classname = null): ?adhoc_task {

        global $DB;

        $concurrencylimit = get_config('core', 'task_adhoc_concurrency_limit');

        $cachedqueuesize = 1200;

        $uniquetasksinqueue = array_map(

            ['\core\task\manager', 'adhoc_task_from_record'],

            $DB->get_records_sql(

                'SELECT classname FROM {task_adhoc} WHERE nextruntime < :timestart GROUP BY classname',

                ['timestart' => $timestart]

            )

        );

        if (!isset(self::$numtasks) || self::$numtasks !== count($uniquetasksinqueue)) {

            self::$numtasks = count($uniquetasksinqueue);

            self::$miniqueue = [];

        }

        $concurrencylimits = [];

        if ($checklimits) {

            $concurrencylimits = array_map(

                function ($task) {

                    return $task->get_concurrency_limit();

                },

                $uniquetasksinqueue

            );

        }

        /*

         * The maximum number of cron runners that an individual task is allowed to use.

         * For example if the concurrency limit is 20 and there are 5 unique types of tasks

         * in the queue, each task should not be allowed to consume more than 3 (i.e., ⌊20/6⌋).

         * The + 1 is needed to prevent the queue from becoming full of only one type of class.

         * i.e., if it wasn't there and there were 20 tasks of the same type in the queue, every

         * runner would become consumed with the same (potentially long-running task) and no more

         * tasks can run. This way, some resources are always available if some new types

         * of tasks enter the queue.

         *

         * We use the short-ternary to force the value to 1 in the case when the number of tasks

         * exceeds the runners (e.g., there are 8 tasks and 4 runners, ⌊4/(8+1)⌋ = 0).

         */

        $slots = floor($concurrencylimit / (count($uniquetasksinqueue) + 1)) ?: 1;

        if (empty(self::$miniqueue)) {

            self::$mode = self::ADHOC_TASK_QUEUE_MODE_DISTRIBUTING;

            self::$miniqueue = self::get_candidate_adhoc_tasks(

                $timestart,

                $cachedqueuesize,

                $slots,

                $concurrencylimits

            );

        }

        // The query to cache tasks is expensive on big data sets, so we use this cheap

        // query to get the ordering (which is the interesting part about the main query)

        // We can use this information to filter the cache and also order it.

        $runningtasks = $DB->get_records_sql(

            'SELECT classname, COALESCE(COUNT(*), 0) running, MIN(timestarted) earliest

               FROM {task_adhoc}

              WHERE timestarted IS NOT NULL

                    AND (attemptsavailable > 0 OR attemptsavailable IS NULL)

                    AND nextruntime < :timestart

           GROUP BY classname

           ORDER BY running ASC, earliest DESC',

            ['timestart' => $timestart]

        );

        /*

         * Each runner has a cache, so the same task can be in multiple runners' caches.

         * We need to check that each task we have cached hasn't gone over its fair number

         * of slots. This filtering is only applied during distributing mode as when we are

         * filling capacity we intend for fast tasks to go over their slot limit.

         */

        if (self::$mode === self::ADHOC_TASK_QUEUE_MODE_DISTRIBUTING) {

            self::$miniqueue = array_filter(

                self::$miniqueue,

                function (\stdClass $task) use ($runningtasks, $slots) {

                    return !array_key_exists($task->classname, $runningtasks) || $runningtasks[$task->classname]->running < $slots;

                }

            );

        }

        /*

         * If this happens that means each task has consumed its fair share of capacity, but there's still

         * runners left over (and we are one of them). Fetch tasks without checking slot limits.

         */

        if (empty(self::$miniqueue) && array_sum(array_column($runningtasks, 'running')) < $concurrencylimit) {

            self::$mode = self::ADHOC_TASK_QUEUE_MODE_FILLING;

            self::$miniqueue = self::get_candidate_adhoc_tasks(

                $timestart,

                $cachedqueuesize,

                false,

                $concurrencylimits

            );

        }

        // Used below to order the cache.

        $ordering = array_flip(array_keys($runningtasks));

        // Order the queue so it's consistent with the ordering from the DB.

        usort(

            self::$miniqueue,

            function ($a, $b) use ($ordering) {

                return ($ordering[$a->classname] ?? -1) - ($ordering[$b->classname] ?? -1);

            }

        );

        $cronlockfactory = \core\lock\lock_config::get_lock_factory('cron');

        $skipclasses = array();

        foreach (self::$miniqueue as $taskid => $record) {

            if (!empty($classname) && $record->classname != self::get_canonical_class_name($classname)) {

                // Skip the task if The class is specified, and doesn't match.

                continue;

            }

            if (in_array($record->classname, $skipclasses)) {

                // Skip the task if it can't be started due to per-task concurrency limit.

                continue;

            }

            if ($lock = $cronlockfactory->get_lock('adhoc_' . $record->id, 0)) {

                // Safety check, see if the task has been already processed by another cron run.

                $record = $DB->get_record('task_adhoc', array('id' => $record->id));

                if (!$record) {

                    $lock->release();

                    unset(self::$miniqueue[$taskid]);

                    continue;

                }

                // Safety check in case the task in the DB does not match a real class (maybe something was uninstalled).

                try {

                    $task = self::adhoc_task_from_record($record);

                } catch (\moodle_exception $e) {

                    debugging("Failed to load task: $record->classname", DEBUG_DEVELOPER);

                    $lock->release();

                    unset(self::$miniqueue[$taskid]);

                    continue;

                }

                $tasklimit = $task->get_concurrency_limit();

                if ($checklimits && $tasklimit > 0) {

                    if ($concurrencylock = self::get_concurrent_task_lock($task)) {

                        $task->set_concurrency_lock($concurrencylock);

                    } else {

                        // Unable to obtain a concurrency lock.

                        mtrace("Skipping $record->classname adhoc task class as the per-task limit of $tasklimit is reached.");

                        $skipclasses[] = $record->classname;

                        unset(self::$miniqueue[$taskid]);

                        $lock->release();

                        continue;

                    }

                }

                self::set_locks($task, $lock, $cronlockfactory);

                unset(self::$miniqueue[$taskid]);

                return $task;

            } else {

                unset(self::$miniqueue[$taskid]);

            }

        }

        return null;

    }

    /**

     * Return a list of candidate adhoc tasks to run.

     *

     * @param int $timestart Only return tasks where nextruntime is less than this value

     * @param int $limit Limit the list to this many results

     * @param int|null $runmax Only return tasks that have less than this value currently running

     * @param array $pertasklimits An array of classname => limit specifying how many instance of a task may be returned

     * @return array Array of candidate tasks

     */

    public static function get_candidate_adhoc_tasks(

        int $timestart,

        int $limit,

        ?int $runmax,

        array $pertasklimits = []

    ): array {

        global $DB;

        $pertaskclauses = array_map(

            function (string $class, int $limit, int $index): array {

                $limitcheck = $limit > 0 ? " AND COALESCE(run.running, 0) < :running_$index" : "";

                $limitparam = $limit > 0 ? ["running_$index" => $limit] : [];

                return [

                    "sql" => "(q.classname = :classname_$index" . $limitcheck . ")",

                    "params" => ["classname_$index" => $class] + $limitparam

                ];

            },

            array_keys($pertasklimits),

            $pertasklimits,

            $pertasklimits ? range(1, count($pertasklimits)) : []

        );

        $pertasksql = implode(" OR ", array_column($pertaskclauses, 'sql'));

        $pertaskparams = $pertaskclauses ? array_merge(...array_column($pertaskclauses, 'params')) : [];

        $params = ['timestart' => $timestart] +

                ($runmax ? ['runmax' => $runmax] : []) +

                $pertaskparams;

        return $DB->get_records_sql(

            "SELECT q.id, q.classname, q.timestarted, COALESCE(run.running, 0) running, run.earliest

              FROM {task_adhoc} q

         LEFT JOIN (

                       SELECT classname, COUNT(*) running, MIN(timestarted) earliest

                         FROM {task_adhoc} run

                        WHERE timestarted IS NOT NULL

                              AND (attemptsavailable > 0 OR attemptsavailable IS NULL)

                     GROUP BY classname

                   ) run ON run.classname = q.classname

             WHERE nextruntime < :timestart

                   AND q.timestarted IS NULL

                   AND (q.attemptsavailable > 0 OR q.attemptsavailable IS NULL) " .

            (!empty($pertasksql) ? "AND (" . $pertasksql . ") " : "") .

            ($runmax ? "AND (COALESCE(run.running, 0)) < :runmax " : "") .

         "ORDER BY COALESCE(run.running, 0) ASC, run.earliest DESC, q.nextruntime ASC, q.id ASC",

            $params,

            0,

            $limit

        );

    }

    /**

     * This function will get an adhoc task by id. The task will be handed out

     * with an open lock - possibly on the entire cron process. Make sure you call either

     * {@see ::adhoc_task_failed} or {@see ::adhoc_task_complete} to release the lock and reschedule the task.

     *

     * @param int $taskid

     * @return \core\task\adhoc_task|null

     * @throws \moodle_exception

     */

    public static function get_adhoc_task(int $taskid): ?adhoc_task {

        global $DB;

        $record = $DB->get_record('task_adhoc', ['id' => $taskid]);

        if (!$record) {

            throw new \moodle_exception('invalidtaskid');

        }